How can I override field validation with a hook_alter_form

Let's say that I want to modify the validation of this field and display a different error message instead of the one shown above, something like "Please type your name" and I want this to happen only when using one particular theme. So I've added the code below into the file "template.php" that resides inside the theme's foder:

function mytheme_form_alter(&$form, &$form_state, $form_id) {
    if($form_id == 'forward_form') {

$form['#validate'][] = 'my_custom_validation';
    }
}

function my_custom_validation($form, &$form_state) {
       if (!$form_state['values']['name'] ) {
          form_set_error('name', t('Please type your name.'));
      }
}








//Implements hook_form_alter for resource under page webform validation error message change function globalfd_form_alter( &$form, &$form_state, $form_id) { if($form_id == 'webform_client_form_83') { unset($form['#validate'][0]); $form['#validate'][1] = 'my_custom_validation'; } } function my_custom_validation($form, &$form_state) { //get value from webform field $phone = $form_state['values']['submitted']['phone']; $email = $form_state['values']['submitted']['email']; //phone number validation if(preg_match("/^\(?(\d{3})\)?[-\. ]?(\d{3})[-\. ]?(\d{4})$/", $phone) || $phone == '') { // $phone is valid } else { form_set_error('phone', t('Not a valid 10-digit US phone number (Please use 1 of the following formats 999-999-9999 or 0001112222 )')); } //email validation if ($email != '') { if (!filter_var($email, FILTER_VALIDATE_EMAIL)) { form_set_error('phone', t('email is not a valid email address')); } }else{ form_set_error('phone', t('email address is required')); } }